Ozone Generator and an Electrode Thereof

ABSTRACT

The inventive ozone generator comprises at least one tubular external electrode ( 4 ), at least one internal electrode ( 7 ), wherein each internal electrode consists of a plurality of tubular metal segments ( 8 ) which are closed at least partially at each end and externally ceramic-coated, said tubular segments are disposed one behind another, mechanically de-coupled from each other and electrically connected, a rod ( 11 ) axially crosses the tubular segments ( 8 ) and is provided on the end thereof with means ( 12, 13 ) for axially clamping the tubular segments to each other in such a way that an electric contact is formed. Each tubular metal segment ( 8 ) is provided at each end thereof with an outwardly convex bottom ( 8   a,    8   b ) which is embodied substantially in the form of a spherical cap, comprises a central area ( 18 ) for electric contact and is provided with a ceramic coating ( 9 ) consisting of at least two thin layers ( 9   a,    9   b ).

The invention relates to an ozone generator of the type which comprises:

-   -   at least one tubular external electrode,    -   at least one internal electrode comprising a plurality of        tubular metal segments which are closed, at least partially, at        each end and externally coated with ceramic, these tubular        segments being positioned one behind the other, mechanically        coupled and electrically linked,    -   means of holding the tubular segments relative to the external        electrode to form an annular discharging interstice,    -   a rod passing axially through the tubular segments and provided        at its ends with means of axially clamping the tubular segments        to each other, to establish electrical contact,    -   means for connecting all the electrodes to an alternating        current source, and    -   means for circulating a gas containing oxygen in the        interstices.

An ozone generator of this type is known, for example, from U.S. Pat.No. 5,145,653. In such a generator, by applying an AC voltage across theelectrodes that satisfies certain limiting conditions, corona dischargestake place in the interstice between the surface of the dielectricformed by the ceramic coating and the external electrode. Thesedischarges cause ozone to be produced. The use of ceramic as thedielectric coating makes it possible to obtain ozone productionefficiencies that are higher than with a glass coating. In particular,the ozone content in the gas leaving the generator can exceed 12% byweight.

However, the ceramic coating is relatively fragile and cracks can appearwhich prevent a good distribution of the surface electric charges, whichcan cause the acceptable voltage limit for the ceramic to be locallyexceeded. This can result in a breakdown of the ceramic coating andcause the generator to be taken out of service.

The mechanical stresses created by the axial clamping of theceramic-coated tubular segments can also contribute to the appearance ofcracks in the ceramic coating.

The object of the invention is, primarily, to provide an ozone generatorwhich no longer, or to a lesser extent, presents the drawbackssummarized above. In particular, it is desirable for the design of thegenerator to allow for a good resistance of the ceramic coating to themechanical and electrical stresses so that the risk of cracks in theceramic coating is significantly reduced, and the efficiency of thedevice is enhanced.

According to the invention, an ozone generator of the type definedpreviously is characterized in that each tubular metal segment isprovided, at each end, with an externally convex end plate, roughly inthe form of a spherical cap, comprising a central area for electricalcontact, and in that the ceramic coating comprises at least two layersdeposited in succession. The assembly exhibits an enhanced resistance tothe mechanical and electrical stresses.

The risks of cracking of the ceramic coating are significantly reduced.

Preferably, the assembly is designed for each internal electrode towithstand, without cracking of the ceramic coating, a torque at leasttwice the rated torque, exerted at the end of the axial rod.

The thickness of each layer is preferably between 50 and 300 μm.

Each layer of the ceramic coating is advantageously deposited by slurrycoating, or powder coating, or plasma spraying.

The external surface of the tubular segments can exhibit a roughness, inparticular in the form of grooves, to enhance the keying of the ceramiccoating. The depth of the grooves or irregularities is advantageously ofthe order of 0.1 mm. As a variant, the outer surface of the tubularsegments can be roughened by sandblasting.

The central area of each end plate of a tubular element comprises anorifice delimited by a cylindrical flange ring projecting externally inthe axial direction, beyond the ceramic coating, relative to the endplate.

Preferably, the external and internal electrodes and the tubularsegments are made of stainless steel.

The invention also relates to an internal electrode, for the ozonegenerator as defined previously, comprising a plurality of tubular metalsegments which are closed, at least partially, at each end andexternally coated with ceramic, these tubular segments being positionedone behind the other, mechanically decoupled and electrically linked, arod passing axially through the tubular segments and being provided atits ends with means of axially clamping the tubular segments to eachother, to establish the electrical contact, characterized in that eachtubular metal segment is provided, at each end, with an externallyconvex end plate, roughly in the form of a spherical cap, comprising acentral area for contact, and in that the ceramic coating comprises atleast two layers deposited in succession.

Apart from the abovementioned provisions, the invention involves acertain number of other provisions that will be more explicitlydescribed below in relation to an embodiment described with reference tothe appended drawings, but which is by no means limiting. In thesedrawings:

FIG. 1 is a diagrammatic lengthwise vertical cross section of an ozonegenerator according to the invention.

FIG. 2 is a vertical cross section seen from the left along the lineII-II of FIG. 1.

FIG. 3 is a larger-scale partial lengthwise vertical cross section,similar to FIG. 1.

FIG. 4 is a larger-scale diagrammatic transverse cross section along theline IV-IV of FIG. 2.

FIG. 5 is a larger-scale partial lengthwise cross section of the wall ofa tubular segment and of its coating.

FIG. 6 is a larger-scale partial transverse cross section of the wall ofa tubular segment, before coating, with grooves on its external surface.

FIG. 7 is a perspective view of the end of a tubular segment of aninternal electrode according to the invention, and

FIG. 8 is a side view of the end of a tubular segment on a scaledifferent from FIG. 7.

Referring to FIG. 1 in the drawings, an ozone generator is shown,arranged in a tank 1, of generally cylindrical form, closed at its ends.The tank 1 comprises, in the vicinity of each of its longitudinal ends,a wall 2, 3 orthogonal to the axis of the tank. Each wall has opposingholes in which are fitted tubes 4 extending from one wall 2 to the otherwall 3. The length of the tubes 4 can be of the order of two meters ormore. The ends of the tubes 4 are welded respectively to the walls 2 and3. The metal tubes 4 constitute the external electrodes connected to theground potential. It can be considered that the set of tubes 4 form asingle external electrode. For reasons of clarity, the number of tubes 4represented in FIG. 1 is limited, but, in practice, the number of tubescan be greater than 100.

The tubes 4 are fixed in a leaktight manner to the inside of the tankand to the walls 2, 3, which are also fixed in a leaktight manner, inparticular by welding, to the cylindrical wall of the tank 1. A coolant,water for example, is used to externally cool the tubes 4. The coolantis introduced through an inlet 5 and drained through an outlet 6.

An internal electrode 7 is positioned inside each tube 4. Each internalelectrode 7 is made of a plurality of tubular metal segments 8, closedat least partially at each end and provided with an external ceramiccoating 9. The tubular segments 8 are positioned one behind the other,in alignment; they are mechanically decoupled and electrically linked bytheir ends that are in contact with each other.

Means 10 of holding the tubular segments 8 relative to the externalelectrode 4 are provided to form an annular discharging interstice Ibetween the external surface of the ceramic coating 9 and the internalsurface of the external electrode 4. The means 10 are formed, forexample, by insulating projections fixed to the internal wall of thetubes 4.

A metal rod 11 passes axially through the tubular segments 8 and exitsat each end. The end sections of the rod 11 are threaded and providedwith a clamping nut 12, 13 bearing against the end plate of the lastadjacent segment 8. One end of each rod 11, the one located on the rightaccording to FIG. 1, is electrically linked to a conductor 14, which isin turn linked to the high voltage terminal of an AC voltage source 15.An insulator 16 surrounds the conductor 14 at its point of entry throughthe wall of the vessel 1.

Each tubular metal segment 8 is provided, at each end, with anexternally convex end plate 8 a, 8 b, roughly in the form of a sphericalcap, which is in particular roughly hemispherical. The radius ofcurvature of the end plate 8 a, 8 b can be different from the radius ofthe cylindrical part of the tubular segment 8.

The central area of the end plate 8 a, 8 b has an orifice 17 for the rod11 to pass through. This orifice 17 is delimited by a cylindrical flangering 18 oriented externally relative to the end plate 8 a, 8 b.

The ceramic coating 9, for a given thickness, is made up of at least twothin layers 9 a, 9 b (FIG. 4). The sum of the thicknesses of the layers9 a, 9 b corresponds to the desired thickness. The layers 9 a, 9 b aredeposited in succession on the tubular segment 8. The number of layers 9a, 9 b can be greater than two, in particular equal to three. Thethickness of each layer 9 a, 9 b is preferably between 50 and 300 μm.

The ceramic coating 9 covers the cylindrical part and the convex endplates 8 a, 8 b of the tubular segment, but the axial end of the flangering 18 projects from the coating 9. Thus, the flange rings 18 of twosuccessive tubular segments 8 can bear against each other and establishthe electrical contact when axially clamped using the rod 11 and thenuts 12, 13.

Advantageously, the external surface of the tubular segments 8 has aroughness that is sufficient to enhance the keying of the ceramiccoating 9.

According to a first possibility, the tubular segment 8 comprises on itsexternal surface, grooves 19 (FIG. 6), typically annular rotation-wiseabout the axis of the segment, the depth of which is of the order of 0.1mm. As a variant, the roughness of the external surface of the segments8 is obtained by sandblasting at adequate compressed air pressure.

The tube 4 and the tubular segments 8 are preferably made of stainlesssteel, as are the walls 2 and 3 and the tank 1.

Means for circulating a gas containing oxygen, in particular air, insidethe generator comprise an inlet nozzle 21 on the tank opening into thespace between the wall 3 and the adjacent end plate of the tank. The gascan circulate in the interstices I to arrive in a chamber delimited bythe wall 2 and the other end plate of the tank. The ozone-enriched gasis discharged through an outlet nozzle 20.

The ceramic used for the coating 9 is chosen to satisfy the followingrequirements:

-   -   strong adhesion to the surface of the steel tubular segment 8;    -   high resistance to mechanical stresses;    -   appropriate thermal expansion coefficient;    -   high dielectric withstand strength, greater than 10 kV/mm;    -   dielectric capacitance that can be adjusted;    -   stability with regard to oxidants.

Among these requirements, the first two are the most difficult tosatisfy.

The adhesion of the ceramic can be easily tested using a hammer tostrike the surface of the ceramic. The resulting configuration of theflakes or slivers should resemble conical holes, the tips of which areformed by steel points on the wall of the tubular segment. Aconfiguration showing greater areas of the steel wall of the tubularsegment clearly indicates inadequate adhesion of the ceramic.

The roughness of the surface of the tubular segment 8, obtained asexplained previously, considerably enhances the keying of the ceramiccoating.

The mechanical strength of the ceramic coating can be verified onassembling the tubular segments 8 in a generator. For this, a torquegreater than the rated torque is exerted on the nuts 12 or 13 located atthe ends of the axial rod 11. For example, the testing torque is greaterthan twice the rated torque. Thus, when the torque of the nuts 12, 13 isdesigned to be of the order of 1 Nm, the mechanical resistance test onthe ceramic coating is performed with a torque of approximately 3 Nm.According to the invention, the tubular segments 8 with convex endplates coated with at least two layers 9 a, 9 b support such a torquewithout cracks appearing in the ceramic coating.

The convex surface, roughly in the form of a spherical cap, of the ends8 a, 8 b of the tubular segments, has a radius of curvature ensuring anoptimal transition for the electrical field. The rounded form of theends 8 a, 8 b makes it possible to avoid excessively sharp corners andexcessively abrupt variations of the electrical field between theexternal electrode and the internal electrode.

A generator according to the invention well withstands the shocks ofpressure and/or voltage and/or temperature. Such shocks appear, inparticular, following instabilities due to the decomposition of theozone when its content exceeds 12% of the gas by weight. For such acontent, when the ozone decomposes, pressure shocks with the release ofheat take place.

The operation of the generator according to the invention is theimmediate result of the above explanations.

The air that enters through the nozzle 21 flows through the intersticesI and is subject to electrical discharges because of the voltage betweenthe internal electrode 7, 8 and the external electrode 4. Ozone isproduced and the ozone-enriched air exits through the nozzle 20.

A generator according to the invention has correctly operated:

-   -   at a high test voltage of 16 kv instead of the 10 kV normally        provided;    -   with charges ranging up to 12 kw/m² instead of 6 kw/m², in        normal operation, to provoke an external insulation fault;    -   with mechanical torques exerted on the nuts 12 or 13 ranging up        to 6 Nm instead of the 2 Nm normally provided.

Despite these extreme operating conditions, no cracks appeared in theceramic coatings of the tubular segments 8.

1. An ozone generator comprising: at least one tubular externalelectrode (4), at least one internal electrode (7) comprising aplurality of tubular metal segments (8) which are closed, at leastpartially, at each end and externally coated with ceramic, these tubularsegments being positioned one behind the other, mechanically decoupledand electrically linked, means (10) of holding the tubular segmentsrelative to the external electrode to form an annular discharginginterstice (I), a rod (11) passing axially through the tubular segments(8) and provided at its ends with means (12, 13) of axially clamping thetubular segments to each other, to establish electrical contact, means(14) for connecting all the electrodes to an alternating current source,and means (20, 21) for circulating a gas containing oxygen in theinterstices, characterized in that each tubular metal segment (8) isprovided, at each end, with an externally convex end plate (8 a, 8 b),roughly in the form of a spherical cap, comprising a central area (18)for electrical contact, and in that the ceramic coating (9) comprises atleast two layers (9 a, 9 b) deposited in succession.
 2. The generator asclaimed in claim 1, wherein it is designed to withstand, withoutcracking of the ceramic coating (9), a torque at least twice the ratedtorque, exerted at the end of the axial rod (11).
 3. The generator asclaimed in claim 1, wherein the thickness of each layer (9 a, 9 b) ofceramic is between 50 and 300 μm.
 4. The generator as claimed in claim1, wherein each layer (9 a, 9 b) of the ceramic coating is deposited byslurry coating, or powder coating, or plasma spraying.
 5. The generatoras claimed in claim 1, wherein the external surface of the tubularsegments (8) exhibits a roughness to enhance the keying of the ceramiccoating.
 6. The generator as claimed in claim 5, wherein the outersurface of the tubular segments (8) is grooved (19).
 7. The generator asclaimed in claim 6, wherein the depth of the grooves (9) is of the orderof 0.1 mm.
 8. The generator as claimed in claim 5, wherein the externalsurface of the tubular segments (8) is roughened by sandblasting.
 9. Thegenerator as claimed in claim 1, wherein the central area of each endplate of a tubular element comprises an orifice (17) delimited by acylindrical flange ring (18) projecting externally in the axialdirection, beyond the ceramic coating (9), relative to the end plate.10. The generator as claimed in claim 1, wherein the external (4) andinternal (7) electrodes and the tubular segments (8) are made ofstainless steel.
 11. The generator as claimed in claim 1, wherein theradius of curvature of the end plate (8 a, 8 b) of the tubular segment(8) is roughly equal to the radius of the cylindrical part of thetubular segment (8).
 12. An internal electrode for the ozone generatoras claimed in claim 1, comprising a plurality of tubular metal segments(8) which are closed, at least partially, at each end and externallycoated with ceramic, these tubular segments being positioned one behindthe other, mechanically decoupled and electrically linked, a rod (11)passing axially through the tubular segments and being provided at itsends with means (12, 13) of axially clamping the tubular segments toeach other, to establish the electrical contact, characterized in thateach tubular metal segment (8) is provided, at each end, with anexternally convex end plate (8 a, 8 b), roughly in the form of aspherical cap, comprising a central area for electrical contact, and inthat the ceramic coating (9) comprises at least two layers (9 a, 9 b)deposited in succession.